The overall hypothesis to be examined in this proposal is that the Insulin Receptor Substrate (IRS) proteins are key mediators of both growth factor and integrin-dependent signaling pathways that promote breast carcinoma progression. The IRS proteins are cytosolic adapter molecules that organize signaling complexes downstream of surface receptors, many of which have been implicated in mammary tumorigenesis. For example, the IRS proteins are essential downstream signaling intermediates of the Insulin-like Growth Factor Receptor (IGF-1R) which has been linked to increased tumor recurrence and reduced survival of breast cancer patients. Moreover, a role for the IRS proteins in signaling by the a6B4 integrin, an adhesion receptor that is also associated with a poor prognosis in breast and other types of cancers, has recently been demonstrated. In fact, there are data to support a synergism between the IGF-1 and a6B4 receptors that involves the IRS proteins. In the first Specific Aim of this proposal, the impact of IRS deficiency on mammary tumor initiation and progression will be examined. MMTV-polyoma virus middle T antigen mice, a transgenic model of breast cancer, will be crossed with IRS-1 and IRS-2 null mice to assess the contribution of these adapter molecules to the ability of polyoma middle T to promote tumor growth and metastasis. In the second Specific Aim, the mechanism of the cooperative regulation of the IRS proteins by integrin and growth factor receptors that have been implicated in breast cancer progression will be investigated. These experiments will address the mechanism by which the a6B4 integrin promotes IRS-1 and IRS-2 phosphorylation and the impact of a6B4 and IGF-1 cooperative signaling on breast carcinoma cell biology. An important implication of this integrin/growth factor cooperativity is that the a6f34 integrin would permit carcinoma cells to respond to IGF-1 signals in environments where the concentration of this growth factor is limiting, such as secondary sites of metastasis. Finally, in the third Specific Aim, the hypothesis that IRS-1 and IRS-2 differ in their downstream signaling properties and, therefore, in their contribution to breast carcinoma cell biology will be investigated. The relative contributions of IRS-1 and IRS-2 to breast carcinoma progression will be examined using in vitro and in vivo assays. In addition, the downstream effectors of IRS-1 and IRS-2 that are essential for promoting breast carcinoma progression will be identified. Taken together, the information gained from the experiments outlined in this proposal will establish a clear understanding of how IRS-1 and IRS-2 contribute to breast cancer and how we can use this information to interfere with breast carcinoma progression.